Throw when `layer(…)` in `@import` is in the incorrect spot (#15109)

This PR throws an error when we notice that an `layer(…)` in an
`@import` or `@media` is incorrect.

This hints the user to ensure that `layer(…)` in an `@import` should be
the first condition. In case of an `@media`, it should be an `@layer …`
instead.

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Co-authored-by: Adam Wathan <adam.wathan@gmail.com>
Co-authored-by: Philipp Spiess <hello@philippspiess.com>